Communication method and common control bus interconnecting a controller and a precision measurement assembly

ABSTRACT

The number and weight of wires interconnecting a host and/or controller with a precision measurement assembly is reduced using a common or shared bus. The bus may be entirely electrical or may include optical fibers to reduce EMI susceptibillty. A custom bus or a known serial network bus such as CAN or SIRCOS may be used.

CROSS-REFERENCE TO RELATED APPLICATION

This as the national stage of International Application No.PCT/IB02/02154, filed Jun. 11, 2002, as amended on Jan. 10, 2003, whichwas published in English Dec. 19, 2002, under International PublicationNo. WO 02/101323 A2 and republished in English Jun. 12, 2003 underInternational Publication No. WO 02/101323 A3 with the amendment of Jan.10, 2003 and which claims priority from U.S. Provisional ApplicationSer. No. 60/297,480, filed Jun. 12, 2001.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to precision measurement tools and, moreparticularly, to a shared control bus between a host and a precisionmeasurement assembly.

2. Discussion of Related Art

Precision measurement tools include coordinate measuring machines (CMM),precision measuring instruments and the like. CMMs range from bench topand articulated arm manual CMMs to high-speed DCC scanning machines,gantries, shop floor measuring robots and horizontal arm CMMs completewith metrology software, probes and accessories with support networks.

For instance, FIG. 1 shows a precision measurement assembly 10 in theform of a vertical CMM, according to the prior art. The assemblyincludes a stationary platform 12 and primary or first a movable part 14mounted thereon. In the case illustrated of a vertical CMM, the firstmovable part 14 takes the form of a bridge which bridges the platform12. Further configurations such as horizontal CMMs, the first movablepart will typically take another form such as simply a vertical column.In any event, for the illustrated case of a vertical CMM, an actuator ismounted either on the platform 12 or the bridge 14 for moving the bridge14 with respect to the platform along an axis such as an axis 15parallel to one of the edges of the platform 12. The bridge 14 may alsotypically have one or more other movable parts such as a second moveablepart or carriage 16 which is actuated by a corresponding actuatormounted on the carriage 16 itself or on the bridge 14. The carriage 16may be moved along an axis such as an axis 17 perpendicular to the axis15. One of these carriages 16 will have a probe 18 mounted thereon forbeing moved along a surface of an object situated on the platform. Aprobe axis 19 may be perpendicular to both the axes 15, 17 and thecarriage 16 may be moved by an associated actuator along the axis 19.The various actuators execute controlled movements of the at least onecarriage 16 and the bridge 14 so as to cause the probe to move relativeto the surface of the object to be measured along a preplanned path.There are a number of position sensors associated with the carriage 16,bridge 14 and platform 12 for sensing the position of the bridgerelative to the platform and the carriage 16 relative to the bridge andfor providing signals having magnitudes indicative thereof. Similarly,the probe 18 is associated with a position sensor which provides asignal having a magnitude indicative of the position of the tip of theprobe 18 relative to the carriage 16, e.g., along an axis 19 of theprobe or carrier.

Various coordinate transformations may be carried out to translate theposition of the probe into a surface map of the object to be measured.The surface of the object is of course known to a large degree inadvance on account of a CAD program or the like stored in a host CPU 22having a display 24 and user input device 26 such as a keyboard and/ormouse. The user will utilize the preexisting CAD representation of theobject to be measured using a software interface program to create thepreplanned path along which the probe 20 is to be moved. The objectiveis to measure the surface of the object with great precision. Such ahost CPU may be connected to the precision measurement assembly directlyor a via a controller 28 containing relays, power supplies, and otherhardware which would not normally be present in a host CPU and whichwould be better situated separate from the precision measurementassembly 10. Such a controller might be connected to the host CPU 22 bymeans of an RS 232 interface 32, one or more twisted pairs comprising anethernet connection 34 and/or any generalized connection symbolized bythe reference numeral 30. One of the disadvantages of having thecontroller separate from the precision measurement assembly 10 is thatnumerous wires 36 have to be utilized to interconnect the controller tothe precision measurement assembly 10, particularly the bridge or otherequivalent first moveable part. In a typical example, there might benumerous temperature sensors mounted on the precision measurementassembly which have to be connected to the controller 28 by as many as20 wires as shown. Similarly, other types of devices such as multiplemotors, servos, encoders, probes and switches may be associated with theprecision measurement assembly and need to be connected electrically tothe controller 28 by means of wires. Also, a power supply within thecontroller 28 has to provide power on additional wires which may be offairly heavy gauge to the precision measurement assembly 10. All thesewires add up to a significant number. In the example shown, a totalnumber of wires of 113 is required. These wires are heavy and have to beenclosed within a flexible conduit called an energy track which isdesigned to smoothly uncoil and coil the wires as the bridge 14 moveswith respect to the platform 12. This energy track is normally situatedon one of the sides of the platform 12 at one end of the first moveablepart 14, in this case the bridge 14.

The large number of wires creates a significant cable drag problem inview of the fact that even the very slightest twist in the bridge causedby such drag will cause a deformation from the mathematical model of theideal system to such an extent that a significant imprecision in themeasurement is introduced. It is also the case that the controlleritself 28 is typically designed in the present state of the artaccording to a fairly obsolete bus architecture (ISA) and it will bedesirable to modernize the controller itself. Another problem is limitedservo performance which it will be desirable to improve.

DISCLOSURE OF INVENTION

An object of the present invention is to reduce cable drag in the energytrack between the controller or host and the precision measurementassembly.

Another object of the present invention is to improve servo performanceand provide a modern bus architecture.

According to a first aspect of the present invention, an apparatus isprovided, comprising (1) a precision measurement tool having astationary platform and a first movable part for moving a probe along asurface of an object situated on the platform wherein an actuator formoving said first moveable part is mounted on said platform or firstmoveable part and plural actuators are mounted on said first moveablepart for moving said probe, (2) a plurality of position sensors mountedon said platform and said first moveable part for providing sensedposition signals having magnitudes which together are indicative of aposition of said probe on said surface, and (3) a controller physicallyconnected to the measurement tool by means of a bundle of signal linesfor communicating actuator control signals from the controller to theactuators and sensed position signals from the position sensors to thecontroller, wherein the apparatus is characterized in that the bundle ofsignal lines comprises a common bus shared by the sensed positionsignals and the actuator control signals.

According to a second aspect of the invention, a method for use incommunicating control and data signals between a controller and a probemovable along a surface of an object for making physical contact withthe surface of the object mounted on a platform part of a measurementassembly and wherein the probe is movable by a first moveable part ofthe measurement assembly, comprises the steps of (1) utilizing a commoncontrol bus interconnecting said controller and a plurality of modulesincluding modules mounted on said first moveable part for communicatingsaid control and data signals over said common control bus by (2)transmitting a command signal from said controller over said commoncontrol bus for use by a plurality of actuator modules to actuatecorresponding actuators for moving said first moveable part and saidprobe in multiple axes according to said command signal for moving saidprobe in continuous contact with said surface along a path on saidsurface for providing a data signal having a magnitude indicative ofsaid probe making contact with said surface at selected points alongsaid path on said surface of said object to a probe module, (3)providing a plurality of sensed position signals from a correspondingplurality of position sensor modules, said plurality of sensed positionsignals having magnitudes indicative of positions of said pointscontacted by said probe along respective axes of said multiple axes ofmovement of said probe, and (4) communicating said data signal and saidsensed position signals over said common control bus from said probemodule and said position sensor modules to said bus for use by saidcontroller in recordation of said sensed data signal.

According to a third aspect of the present invention, an apparatus,comprises (1) a platform for supporting an object mounted thereon formeasurement of a surface thereof, (2) a probe for moving in multipleaxes along a path on the surface of the object for providing a sensedsignal having a magnitude indicative of physical contact with saidsurface as well as positions of said points along said path along anaxis of said probe, (3) a first moveable part movable on said platformfor said moving the probe along at least one of said multiple axes alongsaid path, (4) a probe module, responsive to said sensed signal forproviding said sensed signal to a shared bus according to a preselectedprotocol, (5) a plurality of actuator modules including a correspondingplurality of probe actuators mounted on said first moveable part,responsive to respective probe actuator control signals providedaccording to said protocol on said shared bus, for moving said probe onsaid first moveable part along said path, (6) at least one firstmoveable part actuator, responsive to a first moveable part actuatorsignal, for moving said first moveable part on said platform for saidmoving said probe in said at least one of said multiple axes along saidpath, (7) a plurality of position sensor modules connected to acorresponding plurality of position sensors for providing sensedposition signals on said shared bus according to said protocol havingmagnitudes indicative of positions of said points contacted by saidprobe during movement of said probe along said path, and (8) acontroller for executing a stored program for providing said respectiveprobe actuator control signals and said first moveable part actuatorsignal and responsive to said sensed position signals over said sharedbus according to said preselected protocol for storing said sensedsignals together representing a topological map of said surface.

According further to the first, second and third aspects of theinvention, the sensed position signals are provided over the bus forcomparison to a corresponding one or more of the actuator controlsignals.

In further accord with the first, second and third aspects of theinvention, the shared bus comprises a transmit line and a receive line.The interconnections of the transmit line and the receive line and ismade at least in part by optical fibers.

In still further accord with the first, second and third aspects of thepresent invention, the shared bus further comprises a synchronizationline.

Advantageously, a dramatic reduction in cabling is achieved with theconcomitant increase in measuring performance. Service costs are reducedas a result of using state of the art concepts including fiber optics,distributed/modular controls and digital signal processing. Theinvention also has potential uses in other types of machines besidesthat described.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of a best mode embodiment thereof, as illustrated in theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a prior art precision measurement assembly controlled by acontroller and a host interconnected by a large plurality of wires.

FIG. 2 shows a precision measurement assembly interconnected to a hostusing a common or shared control bus according to the principles of thepresent invention.

FIG. 3 shows a schematic block diagram of the host of FIG. 2interconnected to the precision measurement assembly by means of opticalfibers, according to the present invention, with details of one of themodules shown.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 2, a precision measurement assembly 40 is shownhaving a platform 42, a first moveable part 44, in this case a bridge44, one or more carriers 46 and a probe 48. Not shown but mounted on orwithin the precision measurement assembly 40 are various modules such asthree motor modules for connection to one or more actuator motors each,three encoder modules for connection to one or more encoders each, aprobe module for connection to the probe 48, a limit switch module forconnection to one or more limit switches and a temperature module forconnection to one or more temperature probes. These modules may beinterconnected using a standard bus such as a known or custom serialnetwork bus, e.g., a CAN (Controller Area Network) bus as shown or anonstandard bus. CAN is a known serial bus system with multi-mastercapabilities according to ISO 11898. Using a shared or common bus allowsan interconnection 50 between the assembly 40 and a host 52 to be muchreduced in the number of signal lines required. Although the power linesfor providing power from a power supply 54 to the assembly 40 willremain the same, such as a total of four power wires as shown by a line56, the more than 100 wires required in the bundle 36 of FIG. 1 has beenreduced, for example, to merely three signal lines 58. These can beelectrical wires or optical fibers as shown. The signal lines 50 can beconnected to both the host and the power supply as shown or to the hostonly. Of course, if optical fibers are used, there will beoptical-to-electrical and electrical-to-optical interfaces which are notshown in FIG. 2. In this way, the amount of weight carried in the energytrack on account of signal wires is much reduced and therefore theaccuracy of the precision measurement itself can be improved by reducingany twist caused by changing weight distribution caused by winding andunwinding of the wires in the energy track as the bridge moves withrespect to the platform.

Referring now to FIG. 3, details of the host and one of the modules ofFIG. 2 are shown along with a shared bus, in this case part electricaland part optical. As mentioned in connection with FIG. 2, the precisionmeasurement assembly 40 includes a plurality of modules 60, . . . , 62each of which are connected to at least one device 64, . . . , 66 whichmay be a motor, servo, encoder, probe, limit switch or temperatureprobe, for example. Regardless of the nature of the device, it can beconnected to a bus controller (B.C.) for example the bus controller 68shown in the module 60 of FIG. 3. A connection line 70 is showninterconnecting the bus controller 68 and the device 64 but such a linemay comprise multiple lines to one or multiple devices and may beunidirectional or bidirectional in nature. The bus controller isresponsive to a synchronization signal on a line 72 from a sync bus 74which in this case is a separate sync bus. It is made separate in thiscase because of the importance of the synchronization and timing of thesystem. However, it should be realized that the synchronization signalcould be time multiplexed on the shared transmit and/or receive signallines to be described below.

As known in certain shared or common bus protocols such as the CANprotocol, when data is transmitted, no stations are addressed, butinstead the content of the message is identified by a designation thatis unique throughout the network. The designation defines not only thecontent but also the priority of the message which is important for busallocation among competing stations. Each bus controller within each ofthe modules 60, . . . , 62 in FIG. 3 will include a receive portconnected to a common receive bus (RX) and a transmit port connected toa common transmit bus (TX). These common or shared receive and transmitbuses 76, 78 are shared in common among all the modules 60, . . . , 62of the precision measurement assembly 40 as well as the host 22.

According to the present invention, although the internals of eachmodule will typically involve electrical interconnections using wiresfor the interconnections 70, 72, 74, 76, 78, nonetheless the moduleswill contain electrical-to-optical converters 80, 82, 84 for convertingsuch electrical signals to optical signals and optical-to-electricalconverters 86, 88, 90 for converting received optical signals toelectrical signals as shown. Similarly, the host 22 has a plurality ofconverters including optical-to-electrical converters 92, 94 and anelectrical-to-optical converter 96 for similar purposes. In this way,the common electrical bus can be shared by extension using opticalfibers along much of its length thereby reducing its overallsusceptibility to electromagnetic interference (EMI). This isparticularly useful in the energy track 98 and in the interconnectionsbetween modules 60, . . . , 62. In this way, not only is the weight andsize of the bundle of signals reduced but also the overallsusceptibility of the system to EMI.

Referring now to both FIGS. 2 and 3, it will be realized that thedevices 64, . . . , 66 will include the multiple motors, encoders,probes, limit switches, temperature probes, servos, etc. mentioned aboveand will be connected to various modules 60, . . . , 62 shown in FIG. 3.Among these will be a plurality of position sensors mounted on theplatform and the bridge for providing sensed position signals havingmagnitudes which together are indicative of the position of the probe 48on the surface to be measured. These sensed position signals areprovided on the transmit line 78 to the host 22 for processing therein.The host provides actuator control signals on the transmit line 78 tothe various modules which interpret the signals to decide whether theyare associated with actuators attached to themselves, according to theprotocol utilized such as the CAN bus system suited for networking suchmodules as well as sensors and actuators. Consequently, the commoncontrol bus interconnecting the host and the plurality of modules isutilized for communicating control and data signals as shown foractuating the various actuators for moving the bridge part with respectto the platform and the carrier part 46 with respect to the bridge 44and for receiving sensed position signals from a corresponding pluralityof position sensors having magnitudes together indicative of positionsof the points contacted by the probe 48 along respective axes of themultiple axes of movement of the probe. These sensed data signals arecommunicated over the common bus from the associated module or modulesto the host for purposes of recordation in finally representing atopological map of the surface of the object being measured.

The above principles for carrying out the present invention as describedabove in connection with a vertical coordinate measuring machine (CMM)is also applicable to other types of CMMs including known gantry CMMs,shop floor CMMs thin wall, shop floor CMMs prismatic, etc. as well as toother precision measuring instruments or tools having the same problem.

Although the invention has been shown and described with respect to abest mode embodiment thereof, it should be understood by those skilledin the art that the foregoing and various other changes, omissions andadditions in the form and detail thereof may be made therein withoutdeparting from the spirit and scope of the invention.

1. Apparatus, comprising: a precision measurement tool having astationary platform and a movable part for moving a probe along asurface of an object situated on the platform wherein an actuator formoving said part is mounted on said platform or part and pluralactuators are mounted on said movable part for moving said probe; aplurality of position sensors mounted on said platform and said part forproviding sensed position signals having magnitudes which together areindicative of a position of said probe on said surface; and a controllerphysically connected to the measurement tool by means of a bundle ofsignal lines for communicating actuator control signals from thecontroller to the actuators and sensed position signals from theposition sensors to the controller, characterized in that the bundle ofsignal lines comprises a common bus shared by the sensed positionsignals and the actuator control signals.
 2. The apparatus of claim 1,characterized in that the bundle of signal lines for communicatingactuator control signals and sensed position signals comprises opticalfibers.
 3. The apparatus of claim 1, characterized in that the commonbus comprises a transmit line and a receive line.
 4. The apparatus ofclaim 3, characterized in that the common bus further comprises asynchronization line.
 5. The apparatus of claim 4, characterized in thatthe bundle of signal lines comprising the transmit line, the receiveline, and the synchronization line are optical fiber lines.
 6. Methodfor use in communicating control and data signals between a controllerand a probe movable along a surface of an object for making physicalcontact with the surface of the object mounted on a platform part of ameasurement assembly and wherein the probe is movable by a part of themeasurement assembly, said method comprising the steps of: utilizing acommon control bus interconnecting said controller and a plurality ofmodules including modules mounted on said part for communicating saidcontrol and data signals over said common control bus by transmitting acommand signal from said controller over said common control bus for useby a plurality of actuator modules to actuate corresponding actuatorsfor moving said part and said probe in multiple axes according to saidcommand signal for moving said probe in continuous contact with saidsurface along a path on said surface for providing a data signal havinga magnitude indicative of said probe making contact with said surface atselected points along said path on said surface of said object to aprobe module, providing a plurality of sensed position signals from acorresponding plurality of position sensor modules, said plurality ofsensed position signals having magnitudes indicative of positions ofsaid points contacted by said probe along respective axes of saidmultiple axes of movement of said probe, and communicating said datasignal and said sensed position signals over said common control busfrom said probe module and said position sensor modules to said bus foruse by said controller in recordation of said sensed data signal.
 7. Themethod of claim 6, further comprising the step of using said sensedposition signals in a closed loop control of said movement of saidprobe.
 8. The method of claim 6, wherein said common control buscomprises a transmit line and a receive line.
 9. The method of claim 8,wherein interconnection of said transmit line and said receive linebetween said modules and said controller is made by optical fibers. 10.The method of claim 6, wherein interconnection between said modules andsaid controller is made by optical fibers.
 11. The method of claim 8,wherein said common control bus further comprises a synchronizationline.
 12. The method of claim 9, wherein said common control bus furthercomprises a synchronization line and interconnection of saidsynchronization line between said modules and said controller is made byoptical fibers.
 13. Apparatus, comprising: a platform for supporting anobject mounted thereon for measurement of a surface thereof; a probe formoving in multiple axes along a path on the surface of the object forproviding a sensed signal having a magnitude indicative of physicalcontact with said surface as well as positions of said points along saidpath along an axis of said probe; a part movable on said platform forsaid moving the probe along at least one of said multiple axes alongsaid path; a probe module, responsive to said sensed signal forproviding said sensed signal to a shared bus according to a preselectedprotocol; a plurality of actuator modules including a correspondingplurality of probe actuators mounted on said moveable part, responsiveto respective probe actuator control signals provided according to saidprotocol on said shared bus, for moving said probe on said moveable partalong said path; at least one moveable part actuator, responsive to amoveable part actuator signal, for moving said moveable part on saidplatform for said moving said probe in said at least one of saidmultiple axes along said path; a plurality of position sensor modulesconnected to a corresponding plurality of position sensors for providingsensed position signals on said shared bus according to said protocolhaving magnitudes indicative of positions of said points contacted bysaid probe during movement of said probe along said path; and acontroller for executing a stored program for providing said respectiveprobe actuator control signals and said bridge actuator signal andresponsive to said sensed position signals over said shared busaccording to said preselected protocol for storing said sensed signalstogether representing a topological map of said surface.
 14. Theapparatus of claim 13, wherein said controller is responsive to saidsensed position signals provided over said shared bus according to saidpreselected protocol for comparison to a corresponding one or more ofsaid probe actuator control signals for providing one or morecorresponding error signals to one or more of said plurality of probeactuators.
 15. The apparatus of claim 13, wherein said shared buscomprises a transmit line and a receive line.
 16. The apparatus of claim15, wherein interconnection of said transmit line and said receive linebetween said probe module, said actuator modules, said position sensormodules, and said controller is made by optical fibers.
 17. Theapparatus of claim 13, wherein a portion of said shared bus comprisesoptical fibers interconnecting said probe module, said actuator modulesand said controller.
 18. The apparatus of claim 15, wherein said sharedbus further comprises a synchronization line.
 19. The apparatus of claim16, wherein said shared bus further comprises a synchronization line andinterconnection of said synchronization line between said modules andsaid controller is made by optical fibers.